Genomic instability has been well documented in both cancer cells and precancer cells. Hereditary nonpolyposis colorectal cancer (HNPCC) is caused by mutations in any one of four genes encoding proteins involved in DNA mismatch repair (DMR). Defects in DMR have also been demonstrated in several sporadic cancers as well as precancers, indicating that cellular defects in DMR may be a frequent early step in the evolution of a cancer cell. Genetic analyses of HNPCC kindreds reveal that approximately 25% of the observed alterations in DMR genes predict minor changes in the protein, such as amino acid replacements. With current genetic testing methods, it is not possible to unambiguously assign these sequence variations as either mutations or silent polymorphisms. This research grant application proposes development of functional genetic tests of DNA mismatch repair. This novel technology will have broad utility for basic, clinical and epidemiological cancer research. Defects in DMR predispose to cancer development, both when acquired in a precancer cell through somatic mutation or when inherited as a germline DMR mutation. The technology described in this research proposal will allow assessment of the in vivo function of DMR gene products, and will therefore allow definitive molecular characterization of genomic instability caused by mutations in specific DMR genes. PROPOSED COMMERCIAL APPLICATIONS: Genomic instability is a hallmark of both cancers, precancer cells and inherited predispositions to cancer. Defective DNA mismatch repair is a common source of genomic instability, but current genetic testing methods fail to adequately characterize approximately 25% of the gene variants observed. The technology to be developed in this research, in conjunction with other methods, will allow definitive characterization of cellular DNA mismatch repair competence.